Hydroxy end-terminated linear siloxanes are an important class of silicone additives useful in many commercial silicone compositions. In particular, short chain hydroxy end-terminated linear siloxanes, that is linear siloxanes having a degree of polymerization of less than 10, perform an essential role in most silicone rubber compositions as a plasticizer because of their high ratio of hydroxy groups to dimethylsiloxy units.
However, short chain hydroxy end-terminated linear siloxanes are not easily manufactured because of their inherent propensity to further polymerize by the additional condensation of the terminal hydroxy groups. Thus, short chain hydroxy end-terminated linear siloxanes are difficult, if not impossible, to make by conventional methods such as via the hydrolysis of dichlorodimethylsilane. Such techniques generally produce linear polydimethylsiloxanes with terminal hydroxy groups having a chain length (degree of polymerization) of 10-12, or a SiOH content of 3-4%. A linear polydimethylsiloxane with a SiOH content of 3-4% often does not function optimally as a plasticizer in silicone rubber compositions, where the effectiveness is proportional to the silanol content. Thus, alternative methods of producing short chain short chain hydroxy end-terminated linear siloxanes, having chain lengths of 2-9 and SiOH contents of 8-10% have been described in the art.
U.S. Pat. No. 5,488,125 to Omura et al. discloses a method for manufacturing organosilanes having silanol groups, useful as dispersing agents in silicone rubber compositions. The method disclosed in the '125 patent involves hydrolyzing an organoalkoxy silane, and in particular dimethyl dimethoxy silane, having at most a 20 ppm content of chlorine, with water having an electroconductivity of at least 10.sup.10 M.OMEGA., then further adding a macro-porous cation exchange resin. Similarly, U.S. Pat. No. 5,576,408 teaches a two-stage hydrolysis of organoalkoxysilanes using a macro-porous cation exchange resin and controlling the amount of water in each stage.
U.S. Pat. No. 4,395,563 to Hayes teaches the hydrolysis of alkoxysilanes, and a process for controlling the average chain length of silanol-stopped polysiloxanes. The Hayes patent in particular discloses the preparation of silanol-stopped polydimethylsiloxanes having 2 to 8 siloxane units.
While the prior art methods advantageously control the rate of reaction to provide specific chains of polysiloxanes, these methods also produces substantial quantities of a water/alcoho/siloxane stream that is often treated as a waste-product stream in commercial processes.
The present inventors have discovered a process for preparing hydroxy end-terminated linear siloxanes based on the hydrolysis of chlorosiloxanes. In particular, the present inventors have unexpectedly found a method that avoids or minimizes the further polymerization of the short chain hydroxy end-terminated linear siloxanes, especially with un-reacted chloride terminated polysiloxane reactants. Furthermore, the present invention does not produce high quantities of waste stream products, as does the method of producing short chain hydroxy end-terminated linear siloxanes based on the hydrolysis of the alkoxysilanes.
The object of the present invention is to provide a process for producing hydroxy end-terminated linear siloxanes.